The understanding and control of the interaction of light with matter is fundamental to science and technology. Such an understanding has led to the development of lasers and optical fibers which form the backbone of modern communication systems. In addition, significant advances in nanofabrication techniques over the last few years have made it possible to fabricate sophisticated optical devices with greater functionality at subwavelength dimensions. However, active control of light propagation at subwavelength scales still remains a challenge.
During the last decade there has been tremendous interest in using optical devices based on surface plasmon polaritons (SPPs) for subwavelength control of light. SPPs are collective charge oscillations coupled to an external electromagnetic field that propagate along an interface between a metal and a dielectric. It is the mixed nature of SPPs and their dependence on the index of refraction of the dielectric medium facing the metal which forms the basis for their application in chemical and biological sensing, a technique broadly known as surface plasmon resonance (SPR) spectroscopy. In addition, active control of SPPs has previously been demonstrated by purely mechanical means, electro-mechanical transduction or by manipulating the dielectric refractive index either optically or electrically using liquid crystals, quantum dots, nonlinear optical materials or photochromic dyes. The weak nonlinearity of the metal itself has also been utilized in planar or nanostructured geometries for ultrafast active plasmonic applications. However, the various approaches explored to date to modulate light transmission using SPPs either require large pump fluences (several mJ/cm2), relatively high switching voltages (>10 V), multiple control wavelengths, or achieve only modest values (<70%) of optical switching contrast (defined herein as the change in transmitted optical intensity modulated between its highest and lowest values, normalized to its highest value).
In addition to devices exploiting propagating SPPs, there has also been strong interest in recent years in applications based on stationary charge-oscillation resonances in metallic nanostructures known as localized surface plasmons (LSP). For example, active tuning of LSP resonances has been demonstrated in the case of Au nanoparticles imbedded in a conducting polymer matrix, by electrochemically changing the electronic state of the polymer. Compared to nanoparticle-based devices sustaining stationary LSP resonances, however, metallic cavity waveguide devices sustaining propagating SPPs readily offer the advantages of both deep-subwavelength mode confinement and increased interaction length with active materials.
Although satisfactory in many regards, a need remains for a new strategy by which characteristics of plasmonic structures can be enhanced. And, in particular, a need exists for improved plasmonic structures as utilized in electrochemical applications and devices.